The present invention relates to a ferroelectric RAM (FeRAM) device; and, more particularly, to a memory device that prevents oxidation of a plug caused by oxygen diffusion into the interface between the plug and the bottom electrode and a fabrication method thereof.
A ferroelectric RAM (FeRAM) device is a sort of a nonvolatile memory device using a polarization property of a ferroelectric layer and hysteresis. It is an ideal memory with the advantage of retaining stored information even when the power is turned off, as well as high speed, high capacity and low electric power. As for the dielectric material of the FeRAM, a layer of SrBi2Ta2O9 (SBT), SrxBi2-y(TaiNbj)2O9-z (SBTN), Pb(ZrxTi1-x)O3 (PZT), SrTiO3 (ST) or Bi4-xLaxTi3O12 (BLT) is mostly used. The bottom electrode of a capacitor incorporating the ferroelectric layer is usually formed of Pt, Ir, Ru or an oxide thereof.
To apply the conventional fabrication method of a memory device to the method of fabricating a FeRAM device, it is better to use a ferroelectric layer which has a low crystallization temperature. Also, the contact resistance between the bottom electrode of the capacitor and the plug should be prevented from increasing. Studies on lowering the crystallization temperature of a conventional ferroelectric layer have made much progress, but the technology preventing the increase of a contact resistance between the plug and the bottom electrode is at a standstill. The contact resistance between the plug and the bottom electrode increases, because the polysilicon plug is oxidized during the procedure of a high temperature thermal treatment in an ambient of oxygen for forming a ferroelectric layer. That is, in the subsequent thermal process carried out in an ambient of oxygen for crystallizing the ferroelectric layer, the oxygen diffuses into the interface of the polysilicon plug and the capacitor bottom electrode and oxidizes the surface of polysilicon plug, thus increasing the contact resistance.
FIG. 1 is a cross-sectional view illustrating a structure of a ferroelectric capacitor of a conventional semiconductor device. The semiconductor device of FIG. 1 is formed by a method described below. That is, an inter-layer dielectric layer (ILD) is deposited on the semiconductor substrate 145 with a high doping area 140, and a contact hole connected to the high doping area 140 is formed by selectively etching the ILD. Subsequently, a polysilicon plug 100 is formed by filling up the contact hole with polysilicon, and then a barrier layer 150 of Ti, TiN or TaN is formed on the polysilicon plug 100. After that, a capacitor composed of a bottom electrode 125, a ferroelectric layer 130 and a top electrode 135 is formed on the barrier layer 150.
As described above, the oxygen diffusion barrier layer 150 is formed of Ti, TiN or TaN between the bottom electrode 125 and the polysilicon layer plug 150 to prevent the oxygen from being diffused. However, the barrier layer 150 formed of Ti, TiN or TaN cannot perform its role properly because it loses its characteristics as a diffusion barrier layer at around 500xc2x0 C. Although studies have been done on the barrier metal of a three-element compound such as TiAlN or TaSiN, to prevent the diffusion more effectively, the problem of the barrier layer 150 being oxidized at over 600xc2x0 C. or of the barrier layer structure being destroyed has not been solved yet.
In addition, the structure of the semiconductor device shown in FIG. 1 has a problem that the barrier layer 150 becomes non-conductive, because the bottom electrode 125 and the sides of the barrier layer 150 are exposed during the formation process of a ferroelectric layer 130 and the barrier layer 150 is oxidized while the ferroelectric layer is deposited.
Therefore, researchers are actively conducting research to prevent the oxidation by transforming the structure of a plug. That is, the aim of the research is moving from developing oxygen diffusion materials towards a method for blocking the oxygen diffusion or increasing paths for oxygen.
FIG. 2 shows a structure forming the barrier layer 150 in the contact hole, which is known as a stable plug structure. The reference numerals of FIG. 2 are the same as those of FIG. 1. However, the structure of the semiconductor device shown in FIG. 2 is more or less acceptable in the aspect of anti-oxidation, compared to other existing structures, but this structure as well does not prevent oxygen diffusion effectively. As an example of this, in the case the bottom electrode 125 of a semiconductor device with the structure of FIG. 2 is formed of Pt which causes a lot of oxygen diffusion, and the barrier layer 150 is formed of TiN, the property of the semiconductor deteriorates in a thermal process carried out at around 500xc2x0 C. Moreover, when doing so, the fabricating process becomes more complicated than fabricating the semiconductor device structure of FIG. 1, which leads to an increase in production costs.
It is, therefore, an object of the present invention to provide a semiconductor device, which employs an oxygen diffusion barrier layer inside a plug to protect the plug from being oxidized during a high temperature thermal treatment in an ambient of oxygen effectively, and a fabrication method thereof.
In accordance with an embodiment of the present invention, there is provided a semiconductor device, comprising: an inter-layer dielectric layer formed on a semiconductor substrate with a contact hole inside; a diffusion barrier layer formed at the bottom and on the sides of the contact hole; an oxidation barrier layer formed on the diffusion barrier layer for filling up the contact hole; a bottom electrode of a capacitor contacting the diffusion barrier layer and the anti-oxidation layer; a dielectric layer formed on the bottom electrode; and a top electrode formed on the dielectric layer.
In accordance with an embodiment of the present invention, there is provided a method for fabricating a semiconductor device, comprising the steps of: forming an inter-layer dielectric layer on a semiconductor substrate; forming a contact hole by selectively etching the inter-layer dielectric layer; forming a diffusion barrier layer at the bottom and on the sides of the contact hole; filling up the contact hole by forming an oxidation barrier layer on the anti-diffusion layer; forming a bottom electrode of a capacitor contacting the diffusion barrier layer and the anti-oxidation layer; forming a dielectric layer on the bottom electrode; and forming a top electrode on the dielectric layer.